1. Field of the Invention
This invention relates to an in-focus sensing device, and more particularly to an in-focus sensing device that detects an in-focus state using the image signal.
2. Description of the Related Art
A method of sensing an in-focus state using the image signal has been disclosed in Ishida, et al, "Automatic Focus Adjustment of a TV Camera by a Mountain-climbing Servo System (generally known as the mountain-climbing method)," NHK Technical Report, Vol. 17, No. 1. In FIG. 36 showing the relationship between the frequency component and the lens defocus, the high-frequency components of the image signal increase near the in-focus point as compared with the rest, or the out-of-focus region, and reach at their peak at the in-focus point (this is where the definition becomes maximum). Based on this fact, the system extracts the frequency components from the image signal, and drives the optical system toward positions with higher-frequency components. Many approaches have been proposed which use a plurality of frequencies and switch the frequency to be sensed to higher frequencies near the in-focus point, or weigh the low and high frequencies differently, as disclosed in Published Unexamined Japanese Patent Application No. 55-87114.
Another method of switching the frequency to be sensed on the basis of the information on the optical system (such as the focal length or diaphragm opening) has been proposed as in Published Unexamined Japanese Patent Application No. 60-217759.
Still another method has been disclosed in Published Unexamined Japanese Patent Application No. 2-275916 which moves the optical system to more than two positions, senses the frequency components at those positions, and obtains the ratio of those frequency components to find the in-focus point. FIG. 37 shows the frequency component ratio versus defocus characteristics for the MD curve of FIG. 36 (the curve of the frequency component versus lens defocus characteristics).
As noted above, many methods including a mountain-climbing method have been proposed for in-focus sensing based on a change in the image signal. In the mountain-climbing method, the imaging optical system is driven gradually, and the frequencies to be sensed are switched according to the difference between the frequency components. This approach creates the following problem: as shown in FIGS. 38A and 38B (FIG. 38A shows a case where there are a lot of frequency components, and FIG. 3B shows a case where there are a small amount of frequency components), when the frequency components of the subject is low in frequency, there is a possibility that the frequencies are changed as a result of erroneously determining that the vicinity of the in-focus point has been reached (or erroneously sensing high-frequency components), affecting the in-focus accuracy.
In a method of switching the frequencies to be sensed on the basis of the focal length and diaphragm opening of the optical system, the shape of a mountain to be sensed is made uniform to equalize the response characteristics of in-focus sensing. This method is a type of the mountain-climbing method, so that it has a similar problem mentioned earlier.
In a method of sensing the in-focus point based on the frequency component ratio, the frequency to be sensed is determined for the large defocus (with the object point in the closest position and the optical system in an infinite position, or with the object point in an infinite position and the optical system in the closest position), which results in a small change in the frequency component ratio near the in-focus point, thus decreasing the sensing accuracy. Therefore, it is necessary to prepare a high resolution table with a large capacity.
When the frequencies to be sensed are limited to reduce the capacity of the table, there appears a spurious in-focus point (as indicated by solid lines in FIGS. 39A and 39B), since the frequency component ratio takes the same value as that of the in-focus signal at an optical position other than the proper in-focus point.
In FIG. 39A, the solid lines represent the relationship between the defocus and the frequency component with an optical path difference of d0, with the abscissa indicating the defocus amount and the ordinate the frequency component. In FIG. 39B, the frequency component ratio near the in-focus point A has almost the same value as that near the out-of-focus point B, with the abscissa indicating the defocus amount and the ordinate the frequency component ratio (A: in-focus point; B: appears where S/N ratio is poor due to noise or optical reflection).
In a method of computing the frequency component ratio, when their absolute values are small, there is a possibility that a spurious in-focus point appears owing to computation errors.
Low-frequency components are important in sensing. To assure high accuracy requires the output signal to have a very high S/N ratio. Even when the defocus amount is small (for example, the optical system is in the middle of an infinite position and the closest position), it is necessary to sense frequencies lower than are required, and the signal needs to have a very high S/N ratio to ensure high accuracy.
There is still another method of providing two sensors with a specific optical path difference within the camera body instead of moving the imaging optical system to more than two positions, and obtaining the frequency component ratio from the outputs of those two sensors. This method requires that the relationship between the frequency component ratio and defocus amount as shown in FIG. 37 should be stored in table form for each imaging optical system. Since such a relationship is determined by the optical path difference between the two sensors, when the optical system is installed to another camera body, it is impossible to detectthe in-focus point correctly unless the optical path difference between the two sensors in each camera body is identical design.
Practically, since such an optical path difference should be determined so as to meet the AF function of each camera (with priority given to accuracy or speed), it is not desirable to apply the same value to all cameras. If a plurality of tables of frequency component ratio versus defocus amount for individual optical path differences are stored in the memory of the imaging optical system, there arises a problem of the memory capacity becoming larger.
Generally, in this method, the in-focus point is roughly sensed based on the low-frequency components of the image signal, and then, is accurately determined based on the high frequency components. This frequency switching method, however, needs to memorize at least two types of frequency ratio versus defocus amount tables: a high frequency table and a low frequency table.